Modern computers are formed from a variety of different types of integrated circuit (IC) chips, such as controllers, central processing units (CPUs) and memory. On-chip and chip-to-chip interconnections within a computer are typically made with metal wires. As ICs become more integrated, the wires becomes narrower and more closely spaced. This results in a higher resistance in the wires and a higher capacitance between wires, which act to slow the electrical signal and requires more electrical power. The degree to which the electrical signal is slowed is also proportional to the square of the length of the wire. Such signal delays negatively impact the performance of IC chips and the computer as a whole.
To solve this problem, in-chip and chip-to-chip optical interconnections using light sources and waveguides have been proposed. In an optical interconnection system, an electrical signal from the chip is converted to an optical signal emitted by a light source. The light then travels over a waveguide to a detector, which converts the received light back to an electrical signal. The speed of an optical interconnection is much faster than the flow of electrons in a wire and scales linearly with the length of the optical interconnection.
Such optical interconnection systems generally require an external light source, i.e., one not integrally formed with the IC chip. This is because Si and Si/Ge, the materials presently used to form IC chips, have not been considered suitable for forming integral light sources because they have an indirect bandgap. Instead, external sources with direct bandgap semiconductors, such as vertical cavity surface emitting lasers (VCSELS) formed from AlGaAs/GaAs or strained InGaAs/GaAs quantum-well devices have been used. While these light sources are effective, they need to be separately packaged and interfaced with and aligned to the waveguide, as well as to other devices on the IC chip. This makes for a relatively complicated and expensive on-chip or chip-to-chip optical communication system.
Further complicating chip-to-chip communications is the limited number of contact pads that can be fabricated onto a chip, as well as the limited available chip area. As IC chips increase in sophistication, more and more input/output leads (e.g., pins or balls) are required to accommodate the larger number of bits and inputs/outputs for other applications.
What is needed is a cost-effective optical interconnection system for on-chip and chip-to-chip communication that utilizes a light source and detector formed integral with conventional Si or Si/Ge semiconductor substrates and that is compatible with standard IC fabrication processes.